The slotted slider at the top connects to the frameline selecting lever on the top cover. When you change the lever, a pin on its inside end slides the slider back and forth. A projection on its front end hits the two black diagonal "fingers" visible at the top of the frameline mask; these fingers move the correct set of slots into place.
The parallax compensation slider also moves from side to side. It's controlled by a linkage to the camera's focus coupling arm; this linkage is underneath the rangefinder module. As the slider moves from side to side, a slanted cam on its left end presses against a pin that's connected to the frameline mask assembly. The action of the cam against the pin moves the entire mask assembly diagonally, compensating for parallax.